Electrohydraulic servo control valves are in wide use in many applications such as mobile agricultural and earth-moving equipment, factory robots, hydraulic power presses, pipe, tube and wire-forming equipment, steel and paper mills, chemical processing plants and machine tools of all types. The use of such valves continues to accelerate, primarily due to the decreased cost, miniaturization and increased reliability of computerized and other electronic servo control systems which are used to control such valves.
Servo system equipment is typically quite complicated and expensive. When it malfunctions for any reason, the equipment is often difficult to service and highly trained repair personnel are required to determine the cause of the malfunction. Accurate diagnosis of the malfunction can be very difficult since the automatic feedback loop always attempts to correct for any perceived error between an input signal and the actual output being produced. Thus, it is often impossible to tell by observation whether a malfunction in a servo system is being caused by an erroneous input signal, a bad feedback transducer, faulty electronics, mechanical trouble with the equipment being controlled, problems with the hydraulic system or with the electrohydraulic servo valve.
The analysis of malfunctions is often exacerbated by the fact that the production equipment on which the servo system is utilized is normally very expensive, and a shutdown of such equipment for several tens of minutes to several hours or more is often very costly. For example, an entire steel plant or paper mill may be shut down when one of its key servo systems is malfunctioning. Thus, there has been a continuing need for better preventive maintenance tests and tools which can be used to prevent, spot and/or correct the cause of such difficulties in servo systems before they become major problems. In particular, there has been a continuing need for tests and tools which can be used for trouble-shooting servo valves in the user's factory or at field repair facilities to quickly and correctly identify whether these complicated components are malfunctioning.
It is well-known that the electrohydraulic servo valve is extremely difficult to check for proper operation due in part to its complexity, sensitivity and wide dynamic range. The mechanical and hydraulic components within the servo valve have rather tight mechanical tolerances and are sensitive to wear, minor mechanical damage, and contamination by debris or other foreign matter within the hydraulic fluid having a size greater than about ten microns (0.0004 inch). Prior to the present invention, there was no reliable low-cost test equipment which existed for use at the average user's factory site that we knew of for determining whether the valve is functioning as it was designed to, or if it is in need of repair or adjustment. So most electrohydraulic servo valves suspected of malfunctioning were simply replaced with a duplicate servo valve, and the original suspect valve is sent out to be repaired or rebuilt in whole or in part.
One of several possible reasons for the lack of any good test equipment for analyzing servo valves is that there are many different types of such servo valves, which have a wide range of maximum flow rates and which may be single-stage or double-stage, and in rare instances triple-stage. In the single-stage spool-type valve, the main valve spool is normally actuated directly by an electric torque motor, and fluid is normally ported in a standard four-way configuration. In the two-stage valves, the pilot or first stage receives an electromechanical input, amplifies it and controls the movement of the second or main stage. In a typical two-stage spool-type valve, the low-force torque motor actuates the pilot spool which in turn ports high-pressure fluid to shift the second stage or main spool. Most servo valves are of two-stage design. Also, most servo valves are designed to operate in response to an analog current signal between 0.0 milliamps (mA) and some predetermined maximum current value, such as 10, 40, 100, 200 or 400 mA for example. Normally, the valves are bidirectional and polarity of the input current may be positive or negative. Also, the electric signal may be a simple direct current (DC) signal, or it may be a more sophisticated direct current signal such as relatively high frequency (e.g., 100 to 400 Hz) pulse width modulated (PWM) signal or a basic varying DC signal with a higher frequency alternating current (AC) dithering signal imposed on top of it. The typical factory may have several different kinds of servo valves. So, in order to be useful, any test equipment designed to help trouble-shoot or analyze servo valves should be usable on many different servo valves with different flow ranges and maximum current values.
Recent market research conducted by an affiliate of the assignee of the present invention has shown that approximately 75% of all servo valves sent out for repair require nothing more than simple cleaning and calibration and cleaning. Before the advent of the present invention, such relatively straight-forward repair work could not be successfully performed and tested by the in-house maintenance technicians at the average user's factory. This was because an average factory could not justify spending the money to obtain the expensive, sophisticated hydraulics laboratory equipment, such as a sight flow tube hydraulic test stand, servo amp signal generators or other equipment (like a LVDT-cylinder flow-measuring device) needed to accurately test a rebuilt servo valve for proper operation and calibration. Moreover, these conventional tests are often labor-intensive and time-consuming and require a certain expertise to properly perform them.
When an alleged "malfunctioning" electrohydraulic servo valve is sent out for repair, the charge for examination, tear-down, cleaning and/or calibration can range from about $200.00 (U.S.) and up per valve, even if there is nothing wrong with the valve. Yet, the cost of downtime at a factory or improperly manufactured products is so much higher, that few companies are willing to gamble that nothing is wrong with a suspect valve. So needless repairs and costs are often encountered. Even many companies specializing in servo valve repair lack the equipment for quickly and easily determining whether a valve is functioning according to its manufacturer specifications. Therefore, many such companies routinely tear down any electrohydraulic valve sent in for repair, inspect all components for wear and damage, clean most or all of the components therein using ultrasonic or other suitable cleaning techniques, and then rebuild the valve, and recalibrate it. Thus, it would be very useful to have an affordable, easily used servo valve analyzer which could be used to identify whether anything is indeed wrong with suspect servo valve.
The assignee of the present invention and an affiliated company, namely Servo Craft, Inc. of Rochester Hills, Mich., have worked for several years to develop an electrically controlled servo valve analyzer using conventional electronic flowmeters which is portable and easily and quickly used by repair technicians at a user's factory or a field repair facility to determine whether an electrohydraulic servo valve is in need of repair, what problems, if any, it has, and whether it is properly calibrated. The initial result of this effort was a portable Servo Craft servo valve analyzer, which in a number of respects physically resembled the parts of the servo valve analyzer of the present invention, namely the basic hydraulic circuit, gages and hydraulic power supply shown in FIGS. 1 and 2. It even was constructed in the same kind of portable carrying case. However, this primitive analyzer was very difficult to use and had such limited accuracy that it was often difficult if not impossible to tell whether an observed inaccuracy was the result of a problem with the servo valve or was a result of the fundamental limitations of the analyzer to measure or plot various parameters accurately. Also, it was not a fully automatic test unit. Nevertheless, at least one large automotive company using this analyzer in one of its large plants reported a calculated savings in repair costs of servo valves in excess of $100,000.00 per year.
This primitive servo valve analyzer included two voltmeters with digital read-outs for displaying the input signal current (as measured by the voltage drop across a low-resistance series shunt) and the flow rate through the valve (as measured by a conventional axial-flow turbine flowmeter where the rotating vanes of the turbine were detected by a magnetic pick-up coil). A conventional servo valve amplifier was used to generate the electrical input signal supplied to the servo valve under test. The input to the servo amplifier was supplied by a voltage signal picked off of a manually turned rheostat or a capacitor in a simple RC timing circuit. The current being supplied to the valve and the resulting flow rate through the valve, as measured by the flowmeter, could then be observed on the digital read-outs of the voltmeters. An optional x-y ink-pen plotter, driven in the x and y directions by two external analog signals supplied by the servo valve analyzer, was utilized to produce a "hard-copy" of the results of the test when such a hard-copy was desired. For the plot, the x-axis signal was the current signal being sent to the valve as an input signal, and the y-axis was the resulting flow rate as measured by the flowmeter. However, the plot was unsteady and nonlinear with respect to time. This was because the x-axis signal was either produced by manual rotation of a potentiometer or by the charging or discharging of a capacitor. It was also because the flow rate measured by the conventional electronic flowmeter was inaccurate, especially at low flows. Accordingly, the test results were found to be not repeatable with any real accuracy. For example, we desired to use a plot of the valve when it was properly calibrated as a benchmark against which to compare a plot produced from the testing of the same valve later on to determine whether the valve needed recalibration or was malfunctioning. Due to the inaccuracy and lack of repeatability of the x-y plot, we could not do this.
Most electronically sensed flowmeters used to measure hydraulic flow for tests of servo valves only have an accuracy of about two percent, and are often much more inaccurate in their low hydraulic flow regions. Accuracy of measurement in the very low flow regions is of great importance in evaluating and calibrating the null position of the valve and in observing any nonlinearities in the low-flow regions of valve operation. Many electrohydraulic valves are capable of very accurate operation over two or more orders of magnitude from a very low flow where the valve is barely open to rather high flows where the valve is almost completely open. The foregoing servo valve analyzer was very inaccurate in the low-flow regions due in part to the inaccuracy of its flowmeter. It also was not sufficiently accurate over a wide range to verify proper operation over the wide dynamic range of operation that the valve itself is capable of. However, even the well-equipped servo valve repair facilities that we know of lack an inexpensive and quick way to verify that a cleaned and calibrated servo valve is operating within the manufacturer's original specifications over its full range of dynamic operation.
In light of the foregoing discussion, it is clear that there has been a longstanding need for, and it is the primary object of the present invention to provide, a high-accuracy, easy-to-use, affordable system having manual and fully automatic test modes for analyzing the operation of electrohydraulic servo valves by generating electrical input command signals and monitoring the resulting hydraulic performance of the valve.
Also, in light of the foregoing problems, other more specific objects of the present invention are to provide an electrohydraulic servo valve analyzing system which: (1) can be easily programmed or otherwise adjusted by the user to handle different sizes and types of servo valves; (2) has keyboard means for allowing easy entry of parameters and commands; (3) can accurately and repeatedly generate servo valve command signals with respect to time; (4) employs a high-accuracy electronic turbine flowmeter, and can accurately sense fluid flow over the full dynamic range of the servo valve being tested, including extremely low flows; (5) can accurately and repeatedly generate plots of hydraulic flow rate versus input current data obtained from a valve so as to be able to compare such graphs against one another, and to be able to use such plots as benchmarks; (6) is controlled by a centralized digital control system such as a microprocessor for improved accuracy and reliability; (7) employs a servo valve amplifier and which operates under digital control for improved accuracy with an A/D current feedback loop and repeatability in the DC current signal supplied to the servo valve; (8) is capable of substantially continuous high-speed, acquisition of current and hydraulic flow data resulting from the testing of a servo valve, and has the ability to store or plot same; and (9) can produce x-y plots of input current-output flow rates using an appropriate one of several selectable scales for such x-y plots to suit different servo valves.